A multinational team of astronomers has discovered what happens when a large star accidentally eats a black hole or neutron star: it emits a catastrophically violent, galactic-scale burp that can be detected from over 450 million light years away.
Stellar boffins have previously theorised what would happen in this situation, but now a group based variously in the US, Israel, Canada and Japan have published a paper in the journal Science [behind paywall] which explains how they managed to observe the phenomenon using detective work and information from a number of different instruments and arrays.
The story starts, like Star Wars, long, long ago, in a small, star-forming galaxy far, far away (some 480 million light years). In this case, featuring an object with the catchy name of VT 1210+4956 on the galactic outskirts. The scientists learned from data from a study called the Very Large Array Star Survey (VLASS) that it had started pumping out huge amounts of radio waves, but had not been in another earlier survey using the same telescope array.
Further investigations revealed that an instrument called MAXI (Monitor of All-sky X-ray Image) on board the International Space Station had detected a massive burst of X-rays coming from the same object back in 2014. This burst had lasted just 15 seconds, but during that time its energy emissions had been 10 trillion times greater than those of the Sun.
This was a not-insignificant event.
The astonished astro-whitecoats worked out that the only process which could have created this sequence was a merger-triggered core collapse supernova, aka the stellar cannibalism incident mentioned earlier.
I got you under my skin…
This process can come about because massive stars capable of ending their lives in a supernova are often created in pairs called binary systems, which orbit around each other. In a binary system, one of the stars will inevitably be larger than the other because the universe – or at least, our part of it – is an imperfect place.
Because both stars are very large, their immense mass will make them burn very hot and their lifespans in galactic terms will be comparatively short. Of the two stars, the larger one will burn hotter and use up its nuclear fuel faster, before swelling into a supergiant star and eventually exploding in a supernova. In the case of a merger, things then get very weird.
The remains of the first star, without the nuclear processes required to counteract the immense gravity its mass creates, will collapse into an ultra-dense neutron star or the gravitational singularity of a black hole, phenomena collectively known as compact objects.
We are now left with a massive star approaching the end of its lifespan orbited by a much smaller, but still gravitationally immense, compact object. When the second star swells up into a supergiant, it finds a compact object already there: in fact, the supergiant may increase in size to such an extent that it expands past the orbit of the compact object and the smaller body ends up inside the larger one. What then follows is the worst case of indigestion in the galaxy.
The compact object starts sucking in material from the star’s outer layers, while at the same time the interaction of the compact object’s orbit and the rotation of the star flings massive quantities of it out into space in a huge spiral of gas.
This process continues for a few hundred years, with the compact object moving deeper and deeper beneath the star’s surface, throwing off spiralling streams of gas all the way, until it finally reaches the core.
At this point the interaction between the two binary partners, which has been very energetic but seems oddly sedate in human terms – centuries are not a denomination of time that are often used in reference to stars – suddenly becomes very explosive indeed.
Material from the star’s core interacts with the intruder, creating a superhot disk of material expanding outwards and immense jets of energy and matter blasting out perpendicular to the disk at close to the speed of light. These emissions collide with slower-moving matter around the star with incredible energy, creating a blast of X-rays that you can see from 480 million light years away.
“That jet is what produced the X-rays seen by the MAXI instrument aboard the International Space Station, and this confirms the date of this event in 2014,” said Dillon Dong, a graduate student at Caltech and lead author on a paper in an interview with the National Radio Astronomy Observatory.
While this is happening, the intrusion of the compact object into the star’s core causes it to explode in a supernova almost instantly. The material from that explosion is also ejected outwards at great speed and after a few years it catches up with gases ejected earlier and slams into them.
“The companion star was going to explode eventually, but this merger accelerated the process,” Dong added.
This creates a further burst of radio waves that you can also see from 480 million light years away.
The ultimate result of all this interstellar argy-bargy is a pair of compact objects – black holes, neutron stars, or one of each – orbiting around each other as before, surrounded by an expanding cloud of very brightly shining gas.
A salutary tale which proves that cannibalism is very bad for you and gives you terrible, brightly shining gas which can be seen from 480 million light years away.
“Of all the things we thought we would discover with VLASS,” said Gregg Hallinan of Caltech, one of the paper’s co-authors, “this was not one of them.”
Well, you wouldn’t. But it turns out the galaxy is a cruel, unforgiving and surprisingly windy place. ®